(1) Field of the Invention:
The present invention relates to a magnetic tape recording apparatus, and more particularly to an A.C. biasing structure for a recording magnetic head.
(2) Description of the Prior Art:
In the case of magnetically recording on a magnetic tape with a recording head, an A.C. bias signal having a predetermined magnitude is superposed on a signal current for improving distortion and dynamic range properties. The A.C. bias current is produced by using an A.C. signal generated for erasing the magnetic tape prior to recording. After the amplitude of the A.C. signal is controlled, the amplitude controlled A.C. signal is added to the input signal to be recorded for A.C. biasing.
More specifically, the magnetic tape recording apparatus uses an oscillator for generating the A.C. signal which is applied to the erasing head for erasing signals on the magnetic tape prior to recording. The A.C. signal is also subjected to the amplitude control for the A.C. biasing. The amplitude control is performed by a gain-controlled amplifier which is controlled in accordance with a signal produced by mixing the input signal to be recorded and the amplitude controlled A.C. signal with an adder. The gain-controlled amplifier is controlled to have an optimum gain for obtaining the A.C. bias signal in accordance with the frequency range of the input signal by using a low-pass filter, a peak detector and a comparator. In other words, the amplitude of the A.C. biasing signal is controlled by a feedback loop including the adder, a low-pass filter, a peak detector and a comparator.
Here, the amplitude of the erasing A.C. signal of the oscillator is changed in accordance with the kind of magnetic tape. If a metal tape is used for the magnetic tape, the amplitude of the erasing A.C. signal is made about three to four times as large as that for normal magnetic tape. Thus, the amplitude of the A.C. bias signal is changed in accordance with the kind of magnetic tape. When the amplitude of the A.C. erasing signal is changed, the gain of the gain-controlled amplifier is readjusted to obtain a new value for the optimum effective amount of A.C. bias for recording. This readjustment results from a change in the output from the comparator in the feedback loop which includes the gain controlled amplifier, the adder, the low-pass filter, the peak detector, and the comparator. Thus, the gain of the feedback loop is changed resulting in a variation of response time to the input signal or a variation of the amount of time necessary to return to a steady state when the frequency or amplitude of the input signal is changed. As a result, a tone quality is changed. Thus, the conventional bias control device has a shortcoming that the tone quality differs depending on the kind of magnetic tape to be used.